PEPSI Investigation, Retrieval, and Atlas of Numerous Giant Atmospheres (PIRANGA). IV. High-Resolution Phased-Resolved Spectroscopy of The Ultra Hot Jupiter KELT-20 b

TL;DR

The paper tackles phase-dependent atmospheric properties of the ultra-hot Jupiter KELT-20 b using high-resolution, phase-resolved emission spectroscopy and Bayesian retrieval to derive the dayside P-T profile and elemental abundances. It analyzes five PEPSI datasets (R = $130{,}000$) across pre- and post-eclipse phases to retrieve Fe, Ni, and Ca abundances, including the first emission measurements of Ni and Ca for this planet. The results indicate lower refractory abundances on the pre-eclipse evening side relative to the post-eclipse morning side and point to about 10-30x solar abundances in chemical equilibrium, with Ni/Fe and Ca/Fe remaining consistent with solar within 1-2 sigma and across phases. They find a temperature offset of about 100 K at the top of the thermal inversion between evening and morning hemispheres, underscoring the importance of 3D effects in UHJs. The study also compares emission-derived abundance ratios with transmission results, suggesting that while absolute abundances vary with phase, the abundance ratios of Ni/Fe and Ca/Fe remain relatively constant, highlighting the utility of phase-resolved, high-resolution observations for constraining exoplanet atmospheres.

Abstract

We present five datasets of high-resolution optical emission spectra of the ultra-hot Jupiter KELT-20 b with the PEPSI spectrograph. Using a Bayesian retrieval framework, we constrain its dayside pressure-temperature profile and abundances of Fe, Ni, and Ca, providing the first measurements for Ni and Ca for KELT-20 b in emission. We retrieve the pre- and post-eclipse datasets separately (corresponding to the evening and morning sides, respectively), and compare the constraints on their thermal structures and chemical abundances. We constrain lower abundances in the pre-eclipse datasets compared to the post-eclipse datasets. We interpret these results with an equilibrium chemistry model which suggests ~10-30x supersolar refractory abundances. Due to the well-known degeneracy between absolute abundances and continuum opacities, the abundance ratios are more precise probes of the planetary abundances. Therefore we measure the abundance ratios [Ni/Fe] and [Ca/Fe] across these datasets and find they agree within 1-sigma. We constrain [Ni/Fe] to be consistent with solar within 2-sigma, and [Ca/Fe] to be 0.001-0.01x solar, not accounting for ionization. We compare these abundance ratios with literature results for KELT-20 b in transmission, and find they agree within 2-sigma, suggesting that even though the abundances vary significantly as a function of phase, the abundance ratios of these species remain relatively constant. We find a ~100 K difference in temperature at the top of the thermal inversion, suggesting a hotter evening side than morning side and underscoring the importance of considering 3D effects when studying ultra-hot Jupiters.

Figures (1)

• Figure 1: Signal-to-noise ratio of our observations as a function of orbital phase. Each observation was taken with a 300 second exposure time. The PEPSI red and blue arms are shown as the solid and dotted lines, respectively. The SNR values reported correspond to the 95th percentile per-pixel signal-to-noise ratios for each spectrum. The change from CD II to CD III in the 2023 observations resulted in a significant decrease in SNR in the blue arm.